The invention relates to a frequency modulator circuit arrangement comprising a phase modulator having a first input for a carrier signal, a second input for a modulation signal, and an output for the modulated carrier signal, and an integrator circuit having its output coupled to the input of the phase modulator.
Since frequency modulation is the time differential of phase modulation, a phase modulator circuit can be converted to a frequency modulator circuit by including an integrator circuit in the signal path to its modulation signal input. This fact is well-known; see, for example, the book "Information Transmission, Modulation and Noise" by M. Schwarz (McGraw Hill 1959) section 3-10, FIGS. 3-30. If a fixed frequency source (which may be crystal-stabilized) is connected to the carrier signal input of such a frequency modulator circuit, a variable-frequency signal generator circuit is obtained which can have advantages as far as stability is concerned as compared with a variable-frequency signal generator in the form of a crystal-controlled oscillator in which the frequency controlling property of the crystal is made variable by means of a varactor. However, frequency modulator circuits of the simple phase modulator plus integrator circuit kind are not very suitable for applications in which they would be required to handle quasi or actual d.c. modulation components, for example, in certain forms of signalling systems in which the modulation is in the form of a succession of stationary values. This unsuitability occurs because a d.c. modulation component implies a continuously increasing or decreasing output signal from the integrator circuit and a continuously increasing or decreasing phase shift produced by the phase modulator, and hence in an infinite dynamic range handling capability for the integrator circuit and phase modulator, unless the modulation is chosen so that the integral thereof with respect to time never exceeds (in either direction) values which can be handled by the integrator circuit and phase modulator. Choosing the modulation to satisfy this criterion in such a system tends to result in underutilization of the basic capabilities of the system. It is an object of the invention to mitigate this disadvantage.